JPH1112648A - Steel excellent in buckling resistance and fire resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the steel which is difficult to generate the local buckling even in the case of large width/thickness or without a stiffener against large compression load to be exerted during a large earthquake, and excellent in fire resistance by hot rolling the steel of the prescribed composition, and cooling it in an accelerated manner from the prescribed temperature range to be determined by the composition of the steel at the prescribed speed to realize two- phase structure of ferrite + bainite or martensite. SOLUTION: The steel containing, by weight, 0.03-0.25% C, 0.5-2.0 Mn, and 0.05-0.70 Mo, and as necessary, 0.01-1.0% Si, 0.05-0.50% Cu, Ni and Cr, respectively, and one or more kinds of 0.005-0.10% Nb, V and Ti, respectively, is hot rolled. Then, the steel is cooled from the temperature range of (Ar3 +40)-(Ar3 -80) deg.C at the cooling speed of >=2 deg.C/second. The transformation point is obtained from the formula: Ar3 ( deg.C)=910-310C(%)-80Mn(%)-20Cu(%)-15Cr(%)-55Ni(%)-80Mo(%).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel member used for various buildings in the field of civil engineering and construction, and more particularly to a steel member excellent in buckling resistance and fire resistance during an earthquake.

[0002]

2. Description of the Related Art Steel members having a box-shaped section or an H-shaped section are often used for columns, beams, piers, etc. of steel buildings. Many are manufactured by molding. Steel materials used for these steel members are required to have excellent plastic deformability from the viewpoint of energy absorption during earthquakes,
JP-A-55-119152, JP-A-63-223
No. 123, Japanese Patent Application Laid-Open No. 1-1156422, Japanese Patent Application Laid-Open No. 3-115524, and the like propose a steel material in which the yield ratio is reduced to improve uniform elongation characteristics. In addition, as stipulated in JIS G3136 for rolled steel for building structures that the yield ratio is set to 80% or less, the response from the steel surface to the improvement of seismic resistance is to improve the plastic deformability by the low yield ratio. Is the center.

[0003] Further, in a large-scale earthquake, a large load of repeated tensile and compressive loads may be applied to these steel members, causing local buckling. In addition, cracks may be generated from the buckled places, and tensile deformation after buckling may occur. It can cause brittle fracture and cause serious damage such as collapse of buildings. Even for such local buckling, it is effective to reduce the yield ratio of steel material.
Toyoda, et al., "Effects of Steel Deformation Characteristics on Deformability of Steel Welded Structures," Proc. Of the Japan Welding Society, Vol. 8, No. 1,
p112 (1990).

[0004] In recent years, as steel structures such as buildings and bridges have become larger, they are required to have sufficient seismic performance even in the case of large-scale earthquakes. It is becoming difficult to ensure high seismic performance. For this reason, steel members used for columns and beams, bridge piers, etc. of steel buildings use thicker steel plates and reduce the width-to-thickness ratio of their cross-sections, or reinforce them with stiffening plates, etc. This makes buckling less likely to occur and increases the holding strength.

However, the reduction in the width-to-thickness ratio and the use of the stiffening plate not only increase the cost, but also hinder the degree of freedom in design. Even when the width-to-thickness ratio is large, excellent buckling performance is obtained. Are desired.

[0006] On the other hand, with respect to fires in buildings, a review of fire-resistant design has made it possible to reduce fire-resistant coating using fire-resistant steel having excellent high-temperature strength. There are merits such as the elimination of waste and the expansion of the effective area in buildings, and the application thereof has become active. For construction steel materials with excellent fire resistance, see
JP-A-83821, JP-A-4-56723, JP-A-4-56362, and the like. However, each of them is characterized by a low yield ratio, and as described above, it is difficult to secure sufficient seismic performance by itself.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a large width-to-thickness ratio or no stiffener against a large compressive load acting during a large earthquake. Both are hard to cause local buckling and are excellent in key resistance performance suitable for use on columns and beams of steel structures or piers,
It is another object of the present invention to provide a steel member having excellent fire resistance.

[0008]

The present inventors have conducted intensive studies on the buckling characteristics of a steel member having a box-shaped cross section or an H-shaped cross section, and have obtained the following findings. The buckling characteristics of a steel member are closely related to the stress-strain curve obtained in a tensile test of the steel material used for it. It has been found that performance is greatly improved. Then, in order to obtain such a nominal stress-nominal strain curve, as a result of examining the composition and the manufacturing method of the steel material, after hot rolling, accelerated cooling was performed from a temperature lower than the Ar3 transformation point, and the structure was changed to ferrite + bainite or By forming a two-phase structure of ferrite + martensite, a steel material having the above-described nominal stress-nominal strain curve can be obtained.

That is, the present invention has been made on the basis of the above findings, and the gist of the present invention is that (1) wt.
0.03-0.25%, Mn: 0.5-2.0%, M
o: After hot rolling a steel containing 0.05 to 0.70%, it is determined by the composition of the steel (Ar3 + 40) to (Ar3 +
80) A steel member having excellent buckling resistance and fire resistance, characterized by using steel manufactured at a cooling rate of 2 ° C / sec or more from a temperature range of 0 ° C. (2) By weight%, C: 0.03
-0.25%, Mn: 0.5-2.0%, Mo: 0.0
5 to 0.70%, and further, Si: 0.01 to
1.0%, Cu: 0.05 to 0.50%, Ni: 0.0
5 to 0.50%, Cr: 0.05 to 0.50%, Mo:
0.05 to 0.50%, Nb: 0.005 to 0.10
%, V: 0.005 to 0.10%, Ti: 0.005 to
After hot rolling a steel containing 0.10% or more of one or more, it is determined by the composition of the steel (Ar3 + 40) ~
A steel member having excellent buckling resistance and fire resistance, characterized by using steel manufactured at a cooling rate of 2 ° C./sec or more from a temperature range of (Ar 3 −80) ° C.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each constituent element in the present invention will be described below. C: 0.03 to 0.25% C is an element necessary for securing the strength of the steel member. However, if the content is less than 0.03%, the strength is insufficient. Is impaired, so that the content is 0.1.
03 to 0.25%.

Mn: 0.5 to 2.0% Mn is added to increase the strength of a steel member.
If the content is less than 5%, the strength is insufficient. If the content exceeds 2.0%, the toughness of the base material and the welded portion and the weldability are deteriorated, so the content is 0.5 to 2.0%. I do.

Mo: 0.05-0.70% Mo is an element effective for increasing the strength of steel by improving hardenability, precipitation strengthening, etc., and is extremely effective especially for medium and high temperature strength. It is. However, if it is less than 0.05%, it is difficult to obtain the effect, and even if it exceeds 0.70%, not only the effect corresponding to the addition cost is not seen, but also the weldability is deteriorated. So its content is 0.
05 to 0.70%.

Si: 0.01-1.0% Si is necessary not only to increase the strength of the steel member but also as a deoxidizing agent in the steel making process, but if it is less than 0.01%, the effect is insufficient. If added in excess of 0%, the toughness of the weld is degraded.
0%.

Cu: 0.05 to 0.50% Ni: 0.05 to 0.50% Cr: 0.05 to 0.50% Cu, Ni and Cr are effective in increasing the strength, but each is 0%. If it is less than 0.05%, the effect is not exhibited, and 0.50%
%, The weldability deteriorates, so the content is set to 0.05 to 0.50%.

Nb: 0.005 to 0.10% V: 0.005 to 0.10% Ti: 0.005 to 0.10% Nb, V, and Ti improve toughness and normal temperature and high temperature strength by adding a small amount. Is an effective element, but its content is 0.00
If it is less than 5%, the effect cannot be exhibited effectively,
If it exceeds 0.10%, the toughness of the weld is deteriorated.
The content is made 0.005 to 0.10%.

[0016] In addition, P and S, which are contained as impurity elements, and Al or the like which is added as a deoxidizing agent may be contained, and these elements impair the buckling resistance of the steel of the present invention. It is not something to be done.

Next, the manufacturing conditions will be described. First,
After hot-rolling a steel having the above-described composition, a temperature range of (Ar3 + 40) ° C to (Ar3-80) ° C determined by the composition of the steel is 2
Cool at a cooling rate of at least ° C / sec. Here, the Ar3 transformation temperature is determined by the following equation.

Ar 3 (° C.) = 910-310 C (%)-8
0Mn (%)-20Cu (%)-15Cr (%)-55
Ni (%)-80Mo (%) When the cooling start temperature exceeds (Ar3 + 40) ° C., the structure becomes a bainite single phase structure and the yield ratio increases, so the upper limit of the cooling start temperature is set to (Ar3 + 40) ° C. When the cooling start temperature is lower than (Ar3-80) ° C., pearlite is generated, and a yield shelf is generated in a nominal stress-nominal strain curve.
The lower limit of the cooling start temperature is (Ar3-80) ° C.

The cooling rate is an average cooling rate from the start of cooling to 500 ° C. If the value is less than 2 ° C., the structure becomes ferrite + pearlite, and a yield shelf is generated on the nominal stress-nominal strain curve. Therefore, the lower limit of the cooling rate is 2 ° C./s
ec. Further, even if the cooling rate is 2 ° C. or more, a small amount of pearlite may be generated. Therefore, when more excellent buckling resistance is required, the cooling rate is desirably 5 ° C./sec or more.

[0020]

Embodiments of the present invention will be described below.
After the steel having the components shown in Table 1 was hot-rolled, a 12 mm-thick steel sheet was formed by accelerated cooling. Here, steel types A to E are steels satisfying the component range of the present invention, and steel type F is steel whose Mo is out of the component range of the present invention. Table 2 shows the starting temperature of the accelerated cooling, the difference between the accelerated cooling starting temperature and the Ar3 temperature, and the cooling rate. Here, A-1 to A-4, B-1 to B-4, C-1
To C-3, D-1 to D-3, and E-1 to E-3 are all examples of the present invention satisfying the production conditions of the present invention.
5 to A-7, B-5 to B-7, C-4 to C-6, D-4
D-5 and E-4 to E-5 are comparative examples out of the production condition range of the present invention. F-1 to F-2 are comparative examples in which the production conditions satisfy the range of the present invention, but the components are out of the range of the present invention. Tensile test specimens were sampled from a direction parallel to the rolling direction of these steel sheets, a nominal stress-nominal strain curve was measured by a tensile test, and a minimum gradient from the yield point to a nominal strain of 5% was determined. Table 2 shows the results together with the yield strength at room temperature, tensile strength, and yield strength at 600 ° C.
Shown in All of the examples of the present invention have a nominal stress-nominal strain curve without a yield shelf as shown in FIG. 3 (c), and the yield ratio is 80% or less. Further, the yield strength at 600 ° C. is 2/3 or more of the yield strength at room temperature, and has sufficient performance as fire-resistant steel. On the other hand, the comparative example has a nominal stress-nominal strain curve having a yield shelf as shown in FIG. 3A or 3B, or has a high yield ratio without a yield shelf. Further, in Comparative Examples F-1 and F-2, since Mo is less than 0.05%, the yield strength at 600 ° C. is less than 未 満 of the yield strength at room temperature, and sufficient performance as fire-resistant steel Do not have.

Next, these steel plates are welded as shown in FIG.
A short column compression test specimen having a rectangular cross section shown in Fig. 1 was manufactured. Here, all the width pressure ratios were set to B / t = 30. Further, the longitudinal direction of the short column compression test specimen was made to coincide with the rolling direction of the steel sheet. Then, a compression test was performed by the method shown in FIG. 2, and the strain at which the load began to decrease due to the occurrence of buckling was evaluated as buckling strain. The results of the compression test are shown in Table 2. All of the examples of the present invention have a buckling strain of 1% or more and have excellent buckling resistance. On the other hand, in the comparative example, the buckling strain is small and the buckling resistance is poor because the nominal stress-nominal strain curve has a yield shelf or a large yield ratio.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

As described above, according to the present invention, it is possible to provide a steel material excellent in buckling resistance and fire resistance against a large compressive load received during a large earthquake. It is possible and can be said that it is suitable for use in steel structures such as steel buildings and bridges that require earthquake resistance.

[Brief description of the drawings]

FIG. 1 is a view showing the shape of a short column compression test piece used in a compression test.

FIG. 2 is a diagram showing a setting state of a test machine and a test body in a compression test.

FIG. 3 is a diagram schematically showing a nominal stress-nominal strain diagram obtained by a tensile test.

Claims (2)

[Claims] (1) C: 0.03 to 0.25% by weight,
Mn: 0.5 to 2.0%, Mo: 0.05 to 0.70%
After hot-rolling a steel containing, 2 ° C. from a temperature range of (Ar 3 +40) to (Ar 3 −80) ° C. determined by the composition of the steel.
A steel member having excellent buckling resistance and fire resistance, characterized by using steel produced at a cooling rate of not less than / sec. 2. C: 0.03 to 0.25% by weight,
Mn: 0.5 to 2.0, Mo: 0.05 to 0.70%, Si: 0.01 to 1.0%, Cu:
0.05 to 0.50%, Ni: 0.05 to 0.50%,
Cr: 0.05 to 0.50%, Nb: 0.005 to 0.5%.
10%, V: 0.005% to 0.10%, Ti: 0.0
After hot-rolling a steel containing one or two or more elements of 0.5 to 0.10%, it is determined by the composition of the steel (Ar3 + 4
A steel member having excellent buckling resistance and fire resistance, characterized by using steel produced at a cooling rate of 2 ° C./sec or more from a temperature range of 0) to (Ar 3 −80) ° C.